A sedimentological and geochemical approach to understanding cycles of stratovolcano growth and collapse at Mt Taranaki, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand
The long-term behaviour of andesitic stratovolcanoes is characterised by a repetition of
edifice growth and collapse phases. This cyclic pattern may represent a natural frequency at
varying timescales in the growth dynamics of stratovolcanoes, but is often difficult to identify
because of long cycle-timescales, coupled with incomplete stratigraphic records.
The volcaniclastic ring-plain succession surrounding the 2 518 m Mt. Taranaki, New
Zealand, comprises a wide variety of distinctive volcanic mass-flow lithofacies with sedimentary
and lithology characteristics that can be related to recurring volcanic cycles over >190 ka. Debrisflow
and monolithologic hyperconcentrated-flow deposits record edifice growth phases while
polylithologic debris-avalanche and associated cohesive debris-flow units were emplaced by
collapse. Major edifice failures at Mt. Taranaki occurred on-average every 10 ka, with five events
recognised over the last 30 ka, a time interval for which stratigraphic records are more complete.
The unstable nature of Mt. Taranaki mainly results from its weak internal composite structure
including abundant saturated pyroclastic deposits and breccia layers, along with its growth on a
weakly indurated and tectonically fractured basement of Tertiary mudstones and sandstones. As
the edifice repeatedly grew beyond a critical stable height or profile, large-scale collapses were
triggered by intrusions preceding magmatic activity, major eruptions, or significant regional
tectonic fault movements.
Clasts within debris-avalanche deposits were used as a series of windows into the
composition of previous successive proto-Mt Taranaki edifices in order to examine magmatic
controls on their failure. The diversity of lithologies and their geochemical characteristics are
similar throughout the history of the volcano, with the oldest sample suites displaying a slightly
broader range of compositions including more primitive rock types. The evolution to a narrower
range and higher-silica compositions was accompanied by an increase in K2O. This shows that
later melts progressively interacted with underplated amphibolitic material at the base of the
crust. These gradual changes imply a long-term stability of the magmatic system. The
preservation of similar internal conditions during the volcano’s evolution, hence suggests that
external processes were the main driving force behind its cyclic growth and collapse behaviour
and resulting sedimentation pattern.